The present invention relates to a method for processing antenna patterns of antenna systems for receiving radio waves, more specifically, a method for reducing side lobes in antenna patterns without widening beam widths.
A beam width and a side lobe are indices for describing the characteristics of antenna patterns of general antennas including receiving antennas. Narrower beam widths or lower side lobes give better antenna pattern characteristics.
A multiplicative array is known as a method for reducing beam widths by multiplying each signal received with plural antennas. FIG. 1 shows the constitution of such an antenna which reduces beam widths. In this figure, 101 is a main antenna such as an array antenna comprising plural radiating elements which are arranged along a straight line with equal spaces and 102 is a reference antenna. The reference antenna 102 is arranged apart from the main antenna 101 in the X direction which is the orientation of the beam width to be reduced. 103 is a multiplier which multiplies the signal received with the main antenna 101 by the signal received with the reference antenna 102. In this antenna system having such a constitution, each signal received with antennas 101 and 102 is applied to the multiplier 103 with the same phase to perform the multiplication between the directional characteristic of the main antenna 101 and that of the reference antenna so that the resulting synthesized directional characteristic shows a reduction in the beam width.
The beam width and the side lobe, the indices for the antenna pattern, show antinomical relationship each other. That is, when the beam width is reduced by the conventional method described above, side lobes become high. On the other hand, when side lobes are reduced the beam width becomes wide.
Therefore, for example in radar antennas, when side lobes are reduced, then the beam width is widened and resolution or ability to distinguish targets gets degraded. As a result, the radar could misidentify plural targets as single target. On the other hand, when the beam width is reduced, side lobes become high and the radar might misidentify a target which exists in a certain direction of some side lobes as if it exists in the observation direction although there is nothing in the observation direction.
As described above, the beam width and the side lobe have characteristics conflicting with each other, hence it is impossible to optimize both of these at the same time. Therefore, compromises between the beam width and side lobes have been required such that the beam width is minimized on a certain condition of side lobes, or side lobes are minimized on a certain condition of a beam width as in Chebyshev distribution.